This is a Phase III study to evaluate the drug atosiban (brand name Antocin) for the treatment of preterm labor. Antocin is an inhibitor of the hormone oxytocin which has previously been shown to inhibit uterine contractions in animals. This is a placebo-controlled trial to demonstrate the efficacy of Antocin in preventing preterm delivery and the adverse fetal sequelae associated with preterm delivery. Women in premature labor, who have documented change in the cervical examination and who are between 20 and 34 weeks of gestational age will be considered candidates for the study.